1. Field of the Invention
The ability to measure particles by shape and size is of importance to many groups of people. The food and chemical industries are concerned from a quality control point of view; biologists are interested in characterising cells and monitoring changes in and differences between cells; environmental scientists are concerned with airborne particles and their effect on air quality and health. This list is by no means exhaustive, it is merely intended to illustrate the driving force behind the attempts to develop accurate and reliable measurement instrumentation, and to theoretically understand the nature of the problem.
2. Discussion of Prior Art
There are currently two main optical scattering methods in use in commercially available particle measurement systems. The first method attempts to size particles by measuring their static or dynamic behaviour in fluid. These systems generally measure deposition rate, acceleration in a let stream, or Brownian motion. The second method attempts to size particles by measuring the light scattered from an illuminated particle or ensemble of particles either at a few specific angles or over a large solid angular range. Apart from image analysis systems, none of the commercial instruments is capable of characterising particles by shape, non-spherical particles being sized by assigning an equivalent spherical diameter, although this diameter depends on the measurement method used. What is worse, is that some instruments are known to become inaccurate when tested with non-spherical particles of regular shape, so measurements taken with particles of arbitrary shape have to be treated with some caution.
Instruments which attempt a shape classification are based on image analysis, which requires taking an image of a small number of particles and performing complex image processing. The particle sample has to be prepared beforehand so that it is in a form suitable for image processing, i.e. it has to be processed so that individual particles can be seen with minimal overlapping. Thus there is a considerable time delay before the results are available. The method also requires fast computers in order to do the analysis reasonably quickly. Some of the other instruments also suffer a time delay before measurements are available, and whether this is important depends on the application. It is not necessarily important for batch testing powders for example, but it is of potential importance when monitoring a working environment for asbestos fibres or micro-organisms.
Several commercial laser based instruments are available which will size particles, as disclosed in "Particle Size Analysers Product Roundup", Powder and Bulk Engineering, Feb 1991; pp 42, and other research instruments have been built to investigate various aspects of particle sizing. For example, an instrument has been developed to size particles using the oscillation in intensity of the scattered light, as disclosed in "Drop Sizing by Laser Light Scattering Exploiting Intensity Angular Oscillation in the Mie Regime" by Ragucci, R., Cavaliere, A. and Massoli, P. Particle and Particle Systems Characterisation, Vol 7, 1990; pp 221. Most of the instruments analyze an ensemble of particles and, as stated previously, they assume a spherical particle or particles, and do not give any indication of non-sphericity.
Research reported in "Light Scattering Instrument to Discriminate and Size Fibres Part 2: Experimental System". Particle and Particle Systems Characterisation, Vol 6, 1989; pp 144, has been reported using an instrument designed to discriminate and size fibrous material. In this research, particles are passed through a laser beam in single file using a laminar airflow system similar to the design described below. The forward scattered light is collected by a lens and passed through a polarizing beamsplitter. The intensity of the light in two orthogonal polarizations is then recorded using photo-multiplier tubes. Results show that near spherical particles can be discriminated from fibrous particles by taking the ratio of the polarized intensities, provided that the particle diameter is above 1.5 microns approximately.
An instrument which has been developed to size particles uses the laser Doppler velocimetry technique of "Strengths and limitations of the phase Doppler technique for simultaneous measurements particle velocity and size." by Livesley, D. M. Proceedings-SPIE International Society for Optical Engineering Vol 952, 1988; pp 454, and this has also shown a capability of discriminating near spherical particles from fibrous ones. This technique is based on refraction of rays by the particle, so it is limited to particles larger than 5 microns. The instrument uses two coherent laser beams which interfere with each other, creating a series of fringes in the scattering volume. The spacing of the fringes depends on the wavelength of the lasers and the angle between the beams. Three photo-multiplier tubes are used at different angles of forward scatter which together give an indication of particle speed, size, and non-sphericity. The speed is obtained from the time it takes the particle to traverse from one fringe to the next. The size is obtained from the phase difference in the signals from two detectors, which is a function of speed and particle surface convexity, the rate of sweep of the refracted ray as the particle traverses the fringe being larger for a smaller particle. The third detector allows a second phase difference to be measured, and a difference in the two measured phases is seen when the particles are non-spherical.
We have disclosed in "An instrument for the classification of airborne particles on the basis of size, shape, and count frequency." Atmospheric Environment, Vol 25A No. 3/4, 1991; pp 645 by Kaye, P. H., Eyles, N. A., Ludlow, I. K., and Clark, J. M., and in Applications EP-A-0316171 and EP-A-0316172 an airborne particle classifier (APC) which has some capability of determining particle shape as well as size: it is shown in FIG. 1 of the accompanying drawings and described in detail below. The system is capable of collecting information on a maximum of 10,000 particles per second, and is thus capable of quasi-real time operation. However, the shape information is severely limited because of the small number of detectors, and it is unlikely that it could be used to differentiate unambiguously between different types of non-spherical particle, e.g. fibres and platelets. There is also uncertainty in the trajectory and orientation of particles as they pass through the beam, and it is difficult to determine and allow for the effect of these on the scattering with only three detectors.
US-A-4606636 describes an arrangement where a flow stream carrying particles is carried in a transparent capillary tube along the axis of a non-divergent quadric reflector. A beam of light intercepts the tube at the focus of the reflector, non-reflected scattered rays are intercepted and reflected scattered rays are received on a photosensitive cell or optical scanner which feeds a processing system.
All these prior art systems using quadric reflectors operate on the assumption that the light beam will impinge on a particle at the focus of the reflector. In practice, this is not true. In practice the flow stream will always have a finite thickness and particles carried thereby will not always cross the focus of the reflector. This results in variations of ray path which can result in rays becoming non-monotonic (that is rays scattered at low angles and reflected cross those scattered at higher angles) before being recorded. Images from monotonic and non-monotonic rays are completely different.
Thus no real-time method of shape analysis is yet available, and little investigative work has been done on non-spherical particles.